What's the Deal with Rocket Vibrations?

Vibrations produced during a rocket’s flight have a great impact on its systems.

In my coverage of NASA’s successor to the space shuttle, the
Ares rocket, and the next crew capsule, Orion, I have come across a common
theme: vibrations.

While it may seem like a simple concept, vibrations or
shaking can have a very powerful effect on a rocket’s avionics, hardware, and any
humans onboard. They come from such sources as the thrust of the solid rocket
motors or boosters at lift off and even during flight, the burning of rocket
propellant, and the sheer speed at which the rocket travels–over four times
the speed of sound.

NASA’s Dan Kanigan does a great job of explaining vibrations
and their effect on the Ares I-X test flight
rocket in this blog post.
In addition, he talks about NASA’s solution to the problem.

The vibration that is produced by the burning of the solid rocket propellant
in the first stage booster is called thrust oscillation. These vibrations – or
oscillations – come in the form of waves, which travel up and down the length
of the rocket like a musical note through an organ pipe. One of the biggest
challenges in any rocket design is developing avionics (aviation electronics)
that can function in this vibrating environment.

Vibration is not just a rocket issue, though. All electronic hardware is
tested for its ability to handle shock and vibration. An MP3 player, for
example, has to be tested for its ability to handle the vibrations from someone
walking or jogging while holding it, placing it on a countertop, or
accidentally dropping it on the floor. However, compared to the workout that
Ares I-X’s avionics receive, your MP3 player has got it easy. Imagine shaking
that MP3 player inside an automatic paint can shaker for two minutes while
continuing to play your favorite tunes. That’s kind of what the electronics of
the I-X are up against.

Jon Cowart, the deputy project manager for Ares I-X at
Kennedy Space Center, said in an interview earlier this
year, that gathering data such as aerodynamic data, stresses, strains,
pressures, temperatures, and vibrations is the most important part of the
rocket’s mission, and it will be used to make the Ares I even better.

Yet, the Ares I-X is just one example. NASA has been fighting the vibration issue since Apollo. Then such longitudinal oscillations caused by the burn out of the first stage booster were referred to as the “Pogo effect” and were especially troublesome for Saturn V. During an unmanned Apollo 6 flight, a critical test before manned flights could be approved, vibrations actually caused the main engine to shut down.

The new launch abort
systems being built for NASA’s Orion crew exploration vehicle is facing the
same challenges. When the capsule and
launch abort tower, which sits on top of the capsule, separate, a solid rocket
motor called the abort motor fires, burning the propellant very rapidly to thrust
the vehicle away from the rocket. The plume of hot gases emitted by the motor are the
greatest source of noise and vibrations the vehicle will experience during
flight, says Henri Fuhrmann, program manager of the new launch abort system at Orbital Sciences, an
aerospace company that has partnered with NASA to design and develop the escape
system. Thus the engineers have developed reverse-flow nozzles that turn the flow of the gases 155 degrees, away from the capsule. This will reduce the
acoustics, vibrations, and loads on the capsule that could damage the
electronics and hardware. This is the first time the technology has been developed at this scale, says Fuhrmann. The launch escape system on Apollo had direct flow motors and therefore had to add extra steel structures to increase the distance between the motor and the crew capsule. The Russian’s launch escape system on the Soyuz spacecraft also uses direct flow nozzles.

Kanigan says that the next challenge is to make sure you can control the rocket at such vibrations. For NASA’s new launch abort system engineers built a unique motor called the attitude control motor that steers and controls the capsule. It is also the first of its kind and can fly the vehicle forward and backward, and flip the capsule over to reorient it so the heat shield is facing forward.